The present invention relates to steel cords used as reinforcements for rubber products such as automobile tires and conveyor belts.
Generally, a steel cord of this type is formed by twisting a number of element wires or strands, and a reinforcement includes a number of such cords extending in parallel and coated with rubber. The cord is required to be mechanically strong and chemically adherent to the rubber. The cord should also enable the fluid rubber compound to flow between the wires during manufacture and fill the cavities or spaces among them, so that the wires and rubber form a solid cord.
FIGS. 10 and 11 of the accompanying drawings show two types of conventional steel cords 27 and 28 of "1.times.n" closed strands, "n" being the number of wires. The cord 27 has a single ply of twisted wires 29 (n=3 through 6) for pneumatic radial tires for automobiles. The cord 28 has a bundle of twisted wires 29 (n=7 through 27) for pneumatic radial tires for heavy loads such as for trucks and buses.
The wires 29 are tightly or closely twisted and form a center cavity H (FIG. 10) or cavities H (FIG. 11) among them. When a complex sheet is formed of steel cords and two rubber sheets, the rubber compound is prevented from flowing into the cavity or cavities H, and merely surrounds the outside of the cord. This prevents a complete complex of steel cords and rubber from being formed. Accordingly, in an automobile tire, for example, containing such cords 27 or 28, there is poor adhesion between the cords and rubber, and when the car is driven, the cords and the rubber may separate. This degrades the tire function, and allows water contained in the rubber or penetrating through cracks in the tire to enter the cavity/cavities H. The water may then flow along the cords and corrode them, with the result that they may become mechanically weakened.
In the cord 28 (FIG. 11), the outer and inner wires are twisted in different spirals. When repeated bending stresses are applied on the cord, the inner wires may project between the outer wires, so that the cord may be fatigued.
FIG. 12 shows a conventional open-strand steel cord 30, which is formed by twisting three wires 29 loosely with clearances I between them (see Japanese patent early publication S.55-90692). In order for the rubber compound to sufficiently flow into the inside of the cord between the wires, the clearances I are required to be 0.01 mm or larger. However, the larger clearances result in the following problems.
The free space where the wires 29 can move is enlarged, so that they may deflect and the strand may not be uniform longitudinally. This may cause the cord to buckle when it is loaded with repeated bending stresses.
Because the cord stretches or elongates greatly under very low load, its handling workability is low.
In addition, the tension under very low load applied to the cord when a complex sheet is formed reduces the clearances I, so that the rubber compound may not flow sufficiently into the inside of the cord.